The invention concerns itself with the ball screw art in general, and, in particular, relates to an improved method and apparatus for the return portion of a ball circuit.
Most linear actuator assemblies having antifriction elements are structurally similar. A ball screw is formed with a helical groove or ball path, and a ball nut body is formed with a similar helical ball path through its bore, and a continuous endless circuit of balls is disposed within the nut to provide rolling elements. It is necessary in such ball screw assemblies to provide a means for routing the balls, exiting one end of the nut during their excursion, around to the other end of the nut where the circuit is restarted into the helical ball path.
Many prior art assemblies utilize a transversely drilled hole at each end of the nut which intercepts the ball track, and an external ball return tube is connected between the two holes to provide the return path. The return tube structure creates a non-circular cross-section envelope for the nut which, in many assemblies, makes it difficult to mount the nut without first relieving a machine mounting bore by a milling operation, to accept the ball return tube. An example of such a ball nut is shown in U.S. Pat. No. 3,244,022.
Still other prior art ball screw assemblies utilize a longitudinal return hole through the nut body, parallel to the central axis, where the balls are fed into the nut body return hole at one end and returned to the entrance, or beginning helical ball path portion of the nut. These return hole assemblies require end closures for the nut in order to turn the ball circuit in the opposite direction. These recirculating ball screw assemblies may use a nonmetallic end closure cap which is grooved at the interface between the nut and cap to provide a channel for the ball return circuit. An example of this type of structure is shown in U.S. Pat. No. 4,074,585, in which a single ball nut has a pair of end closures which are grooved to guide the balls at the end face of the nut. The structure shown in this patent requires the relief of the end of the return hole on the nut, to minimize jamming of the balls at the interface. The closure shown is bolted tightly to the end of the nut and is thus constrained from free movement with respect to the nut. Additionally, the end closure is counterbored slightly so that it may be centered accurately on the ends of the nut in assembly. A difficulty arises in use of a nut of this sort, since the plastic end closure is very active in response to thermal changes. Thus, as temperature changes are experienced by the end closure, dimensional changes may shift and distort the groove with respect to the nut and may create a condition which jams the balls. Once the balls have jammed at the interface of the two members, skidding will immediately take place and the balls, nut and/or screw may be seriously damaged. Another difficulty may be encountered when using a structure similar to the reference patent in that it is difficult to connect the assembly to a similar assembly in order to provide a backlash-free system. Back-to-back assemblies would result in relatively spongy plastic members absorbing much of the load, thereby resulting in a low static stiffness assembly.
Applicant has obviated the difficulties inherent in the prior art assembly by a structure which utilizes a return hole through the nut body, in combination with an end closure device which controls the ball circuit through approximately one revolution around the screw out of the nut, at which point the balls are received in an internal passageway located within the nut closure, the passageway being formed of a first portion generally tangential to the helical ball path, and a second portion which is in line with the return hole of the nut. The first and second portions are blended smoothly into one another to govern the balls through a turn around path.
It is therefore and object of the present invention to provide an improved linear actuator assembly which is relatively jam-proof at the ball nut return points.
Another object of the present invention is to provide an end closure assembly for a linear actuator which is relatively jam-proof during ambient thermal excursions.